The telephone communication system has been widely spread nowadays, and there are increasing cases that one person uses a plurality of telephones. When one person uses a plurality of telephones, he/she should sometimes operate plural telephones at the same time. For example, in a situation where an extension telephone of an office and a cellular phone are both usable, if there is an incoming call on the cellular phone while a user is on the extension telephone, it is necessary for him/her to perform an operation of checking a caller by taking a look at the screen of the cellular phone for deciding whether he/she should answer the call. Furthermore, when he/she decides to answer the call on the cellular phone as a result of the check of the caller, it is necessary for him/her to perform an operation of answering the call on the cellular phone after performing an operation of putting the call on the extension telephone on hold.
Japanese Laid-Open Patent Publication No. H10 (1998)-240656 discusses an incoming call controlling method in which a user may check an incoming call on an extension telephone while operating a personal computer terminal by linking a private branch exchange with a server and informing the personal computer terminal of an incoming call on the extension telephone.